The Light of Thabor

"When thou art alone in thy cell, shut thy door, and seat thyself in a corner; rise thy mind above all things vain and transitory; recline thy beard and chin on thy breast; turn thy eyes and thy thought towards the middle of thy belly, the region of the navel; and search the place of the heart, the seat of the soul. At first, all will be dark and comfortless; but if you presevere day and night, you will feel an ineffable joy; and no sooner has the soul discovered the place of the heart, then it is involved in a mystic and etherial light."

Note that if one 'places the beard and chin on one's breast and turns one eyes and thought towards the middle of one's belly, the region of the navel,' it has approximately the same effect as placing the mind (sati) around the mouth while sitting upright with the chin tucked in.

Rising above all things vain and transitory = separating one self from the diversions (Nīvaraṇā). I am not saying the two practices are the same in intent or consequence, but the form of the meditation practice of these monks is virtually identical to the initial steps in jhāna practice.

As a follow-up, this practice, which was the exclusive manner of prayer of a small group of monks called 'Quietists', was discovered by the wider Christian community, brought into the discussion of the substantiality of God (was this light material or immaterial and depending on that was this a legitimate vision of God or not) and became a raging debate among the people of Constantanople as well as among the ecclesiastics  (from the Buddhist perspective, bringing an intermediate result of a practice of letting go of the world into a discussion of existence or non-existence). After a great ruckass it was finally decided that it was a legitimate view of the eternal light of God. Gibbon, in his really witty way, ridicules the whole idea without having any personal experience of the practice: "and, after so many insults, the reason of mankind was slightly wounded by the addition of a single absurdity." And the educated English of the world lost an opportunity to discover jhāna practice, for Gibbon was sacred wisdom right up until the end of WW I, where wisdom was abandoned altogether in the West.